This invention relates to a heat exchange device and more specifically to a furnace, boiler or the like which transfers heat from a burning fuel to a working fluid contained within a conduit. The working fluid may be air, water, oil, brine or any other suitable similar fluid. Accordingly, the invention has utility as the furnace or hot water supply of a home but is not so limited, having application for a wide variety of heat exchange applications.
Known heating devices of this nature normally include a combustion chamber around or through which conduits pass such that working fluid may be directed through the conduit so as to absorb heat from the combustion chamber or the walls thereof. Inasmuch as heat passes through the walls of such conduits in proportion to the temperature gradient thereacross, it would be desirable to establish a maximum temperature at the conduit surface adjacent the combustion chamber so that heat may be most rapidly transferred to the working fluid. Normally, the heat exchange conduits in furnaces or boilers of this type comprise metal chambers, tubes and the like which if subjected to constant high heat in the presence of oxygen and products of fuel combustion would rapidly burn up and degrade, especially if contacted by that portion of the flame known to produce the highest temperature point therein, i.e. at the tip of inner reducing cone of the flame. It has been found that when burning gaseous hydrocarbon fuels such as natural gas, this temperature reaches approximately 2400.degree. F. At such high temperatures, rapid oxidation of the heat exchange conduits would occur and this has detered the use of otherwise maximum temperature gradients which would result in maximum heat transfer rates at such conduit surfaces. Accordingly, direct flame contact with such conduit surfaces is avoided, except in industrial applications where very expensive corrosion resistant alloys such as Inconel or Hastalloy can be used.
An additional phenomena which has restricted the use of direct flame impingement upon working fluid conduits in heat exchange units of this type is that the presence of the much cooler working fluid within the conduit tends to reduce the temperature of the flame such that the gases in the region of the highest temperature point of the flame need additional residence time for the combustion process to be completed. In other words, the continual movement of the working fluid through the conduit tends to quench and accordingly slow down the combustion reaction.
A common constructional feature of prior art heaters is that they are operated by natural convection, that is, the buoyancy of the heated combustion gases cause them to rise which in turn enables fresh combustion air to be drawn into the flame region to sustain burning. Also, precise control of the volume of combustion air utilized is thus not available. Generally, systems of this type are operated intermittently, that is, gases are burned upon signal, effective as when the working fluid drops below a predetermined temperature level and shut off when the temperature thereof reaches a desired level. Accordingly, when fuel is not being burned, natural convection will draw heated air from the combustion chamber and adjacent heat sinks upwardly through the exhaust stack to the atmosphere. This loss of heat from the walls of the combustion chamber and other parts of the equipment during the off cycles of such devices, reduces overall system efficiency.
It is accordingly an object of the present invention to present a device of such construction that it may be operated so as to take advantage of maximum feasible temperature gradients in order to achieve a high heat transfer rate while avoiding the aforementioned prior art drawbacks. This and other objects of the present invention are accomplished by the use of a device comprising a housing having at least a first exchange manifold which forms the lower part of a combustion chamber disposed directly thereabove. The top surface of the manifold includes or supports a working fluid conduit. A flow pattern is established in the combustion chamber by the physical relationship of the burner relative to the point of maximum negative pressure, such that the flow of hot combustion gases is tangential to the working fluid conduits. This tangential flow enables a physically optimal relationship to be established between the working fluid conduits and the flowing gas stream, such that a maximum heat transfer rate is established without direct impingement of the hottest portion of the flame on the metal surface. This optimal relationship depends upon physically structuring the surface which is being brought into contact with the tangential stream of flowing hot combustion gases and locating the surface relative to said stream at such a point that maximum turbulence is established. It is found that, by means of such a device, the tendency of the cool surface of the working fluid conduits to quench the combustion process is avoided completely. The application of the device is, however, limited to the use of 300 series stainless steels or any more corrosion resistant alloy for the material of construction of the working fluid conduits, or to the use of the steel surface, which has been treated with a thin film of ceramic material or a fused metal oxide film of such material as will not be fused or vaporized at temperatures of 2600.degree. F. or below. Alumina is an example of such a material. Also, in some cases, the object could be achieved by the construction of the conduit from highly corrosion and heat resistant materials, e.g., inconel (a trademark of International Nickel), and hasteloy (a trademark of Haynes Stellite Co.), which are high chromium-nickel alloys. In any event, the physical geometry described above enables a flame resulting from the burning of a gaseous fuel, such as oxi-hydrogen, carbon monoxide, various hydrocarbon fractions, or other similar materials to be directed downwardly towards an indirect contact therewith. Means are also included for introducing a pressure gradient, directed from the top to the bottom of the housing so that the combustion gases are downwardly directed over the manifold prior to their exhaust from the housing. This pressure gradient is normally accomplished by introducing a negative pressure below the heat exchange manifold in the housing, but could also be created by applying a positive pressure to the top of the housing above the heat exchange manifold. The burning cycle is conducted intermittently dependent upon the temperature of the working fluid and the means for introducing a pressure gradient in the housing is operational only during such burning cycle.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.